Experimental investigations on heat transfer enhancement of enhanced surfaces in spray cooling using HFE-649

Abstract

The present study utilizes commercial HFE-649 fluid to analyze the effect of enhanced surfaces on spray cooling. Two pressure swirl nozzles, N1 and N2 with an orifice diameter of 0.51 mm and 1.19 mm respectively, have been used to generate micro-sized droplets and to diffuse these droplets on copper specimen of 10-mm2 cross-section. The volumetric flux range of the fluid varies from 1.00 to 21.67 cm3/cm2s. Three enhanced surfaces, namely, two micro-coated surfaces having coating thickness of 100 µm and 200 µm, and a micro-finned (1 mm fin height) surface, have been used in the present study. Heat transfer enhancement of these enhanced surfaces has been compared with a reference plain surface. With micro-coated surface having thickness 100 µm, the maximum heat flux and heat transfer coefficient of 115 W/cm2 and 9.87 W/cm2K with N1, and 143.48 W/cm2 and 10.31 W/cm2K with N2 has been achieved, respectively. When compared to plain surface, coating thickness of 100 µm resulted in a maximum enhancement in heat flux of 61.54% with N1 and 52.91% with N2, and enhancement in heat transfer coefficient of 352% with N1 and 326% with N2. Effect of subcooling at two fluid inlet temperatures, 25℃ and 49℃, on heat transfer performance of enhanced surfaces has also been investigated. The critical heat flux also increased by up 97.1% for the micro-coated surface having 100 µm thickness at near saturated condition when compared to the plain surface, depending upon the volumetric flux. A new correlation for calculating HTC and CHF has been proposed which is based on the earlier correlation proposed by Estes and Mudawar for plain surface. In addition, a correlation to estimate CHF for enhanced surfaces has also been proposed in terms of density ratio, Weber number and Jakob number.